Abstract

The 320 EeV air shower detected by the Fly's Eye poses and important
problem. Careful analysis of path-length limitations for the possible
particle types due to cosmic background radiation verifies that the
particle very likely traveled less than 50 Mpc from its source. The best
candidates for accelerating particles to such high energies are the very
powerful radio galaxies; however, they are all more than 100 Mpc
distant. Our search finds no likely source within 50 Mpc in the
direction from which the particle arrived. This prompts consideration of
less likely astrophysical sources, like M82, as well as nonstandard
mechanisms like cosmic string annihilation. It is also conceivable that
the air shower was produced by some nonstandard particle whose path
length is unlimited because it does not interact with the cosmic
background radiation. A less radical alternative is that relatively
strong magnetic fields deflected the particle's path through a large
angle, so it could have originated at a nearby radio galaxy at an
earlier time of strong activity.

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